Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/14—Relay systems
  • H04B7/15—Active relay systems
  • H04B7/185—Space-based or airborne stations; Stations for satellite systems
  • H04B7/1853—Satellite systems for providing telephony service to a mobile station, i.e. mobile satellite service
  • H04B7/18532—Arrangements for managing transmission, i.e. for transporting data or a signalling message
  • H04B7/18534—Arrangements for managing transmission, i.e. for transporting data or a signalling message for enhancing link reliablility, e.g. satellites diversity

Definitions

• the present invention relates to broadband satellite communications and, more precisely, to beamforming techniques for broadband satellite communications.
• MIMO Multiuser technology outstands as one of the major techniques to improve the performance of the wireless communications, being recently proposed for IEEE 802. Hn, IEEE 802.16e and UMTS-HSDPA. This technology provides large data rate while using the same amount of spectrum and power, thanks to its spatial multiplexing capabilities.
• MIMO Multiple Input Multiple Output
• the MIMO application requires for more than one path to make the information reach a Satellite Terminal (ST) on the Earth.
• ST Satellite Terminal
• current commercial systems are targeted to create a single communication path to deliver the information to each ST, making use of frequency planning and/or sharper satellite beams, to avoid interference among the serviced STs.
• frequency planning and/or sharper satellite beams to avoid interference among the serviced STs.
• an alternative scenario is obtained when the frequency reuse is reduced to one and information to one satellite terminal is delivered through multiple satellite beams simultaneously.
• a ST may receive signals from several satellite beams. Therefore, MIMO channels can be obtained if a joint processing is carried over all the beams. L.
• MMSE Minimum Mean Square Error
• the present invention aims to solve the above- mentioned problems by means of a method of assigning precoding gain which optimizes the satellite scenario characteristics and multiuser diversity, providing a high system performance while requiring a low complexity design.
• the current method applies MIMO techniques to the user segment of the forward link (FL) in a satellite system under the control of a single terrestrial gateway.
• a scheduler in the gateway runs a multibeam opportunistic beamforming (MOB) technique, wherein the satellite antennas radiation pattern, the channel Line of Sight
• MOB multibeam opportunistic beamforming
• the large number of available users or STs in a satellite scenario allows the possibility of obtaining a multi-user gain (through the application of the MIMO multi-user transmission techniques) .
• the Multibeam Opportunistic Beamforming scheme presents several advantages, such as its low complexity design, high system performance, users selection process and its application not only in systems with full channel knowledge, but also to systems with partial channel knowledge .
• the broadband satellite system comprises a multibeam satellite having a plurality of antennas and a plurality of satellite beams, a gateway and a number N of satellite terminals, and wherein the gateway is configured for processing and serving a plurality of K beams of that plurality of satellite beams towards that number N of satellite terminals, wherein K ⁇ N, and wherein that assignation of a precoding beam forming scheme comprises a joint processing over that plurality of K beams.
• That precoding beam forming scheme is dependent: on the radiation pattern of the satellite antennas; on the line of sight channel characteristics and on the multiuser diversity caused by that number N of satellite terminals.
• dependence of the precoding beam forming scheme on the multiuser diversity comprises a selection of K satellite terminals among that number N of satellite terminals, wherein K ⁇ N, that selected K satellite terminals having best signal-to-noise ratio or signal-to-noise-interference-ratio.
• That selection of K satellite terminals is carried out according to the following steps: dividing the area in which the number N of satellite terminals is located into a number K of cells, each of them comprising at least one satellite terminal; at each cell, selecting one satellite terminal corresponding to a best value of signal-to-noise ratio or signal-to-noise-interference- ratio.
• That joint processing and selection of K satellite terminals over the plurality of K beams is carried out following the steps of: generating a first precoding vector and transmitting it towards a first cell from the satellite, wherein that first precoding vector is built from the information of the radiation pattern of the satellite antennas, that precoding vector being a vector KxI and comprising the contribution of each of the K beams over that first cell; feeding back to the satellite a signal-to-noise ratio measured by each of the satellite terminals in the cell; at the satellite side, selecting a satellite terminal within said cell having best signal- to-noise ratio and requesting said selected satellite terminal to send its channel characteristics; building subsequent precoding vectors, selecting satellite terminals having best signal-to-noise-interference ratio within corresponding cells and requesting each of said selected satellite terminals to send its channel characteristics, each subsequent precoding vector being dependent on the information of the radiation pattern of said satellite antennas and on the channel characteristics of the previously selected satellite terminals, said precoding vector being a vector KxI and comprising the contribution of each
• That first precoding vector is the result of normalising in power a first vector which comprises K power contributions corresponding to said satellite beams towards said cell
• said subsequent precoding vectors are the result of normalising in power a corresponding vector which comprises K power contributions corresponding to K satellite beams towards a cell.
• That subsequent precoding vector is dependent on a blocking matrix having size Kxp .
• the channel characteristics comprise both amplitude and phase information.
• the joint processing over said plurality of K beams is carried out following the steps of: building a first precoding vector, wherein said first precoding vector is built from the information of the radiation pattern of said satellite antennas, said precoding vector being a vector KxI and said precoding vector comprising the contribution of each of said K beams over said first cell; building K-I subsequent precoding vectors, each of them having size KxI, each subsequent precoding vector being dependent on the information of the radiation pattern of said satellite antennas and on the precoding vectors previously built and said precoding vector comprising the contribution of each of said K beams over a corresponding cell; building a precoding matrix having size KxK, wherein the K columns of said precoding matrix are the K precoding vectors previously built.
• That subsequent precoding vector is dependent on a blocking matrix.
• This method further comprises: transmitting said precoding matrix towards said area; at each cell, feeding back to said satellite a signal-to-noise-interference ratio measured by each of the satellite terminals in said cell; selecting one satellite terminal per cell, said selected satellite terminals having best signal-to-noise- interference ratio within corresponding cells.
• the present invention provides a method of simultaneous transmission of data to K satellite terminals in a broadband satellite system, said broadband satellite system comprising a satellite and a number N of satellite terminals wherein, prior to starting said simultaneous transmission of data to K satellite terminals, the method comprises the step of: assigning a precoding beam forming scheme and selecting a group of K satellite terminals according to the method previously described.
• a computer program comprising computer program code means adapted to perform the steps of the mentioned method when the program is run on a computer, a digital signal processor, a field- programmable gate array, an application-specific integrated circuit, a micro-processor, a micro- controller, or any other form of programmable hardware.
• Figure 1 shows a satellite system according to an embodiment of the present invention.
• Figure 2 shows a radiation pattern of the satellite antennas according to the present invention.
• Figure 3 shows a scheme representing a computation of a radiation angle according to the present invention.
• forward link refers to a communications relay satellite link from a fixed location (e.g., a gateway) to a user. Such a link comprises both an uplink (gateway to satellite) and a downlink (satellite to user) .
• the general term "user” refers to both mobile users (such as mobile terminals) and fixed ones (such as a satellite terminal) .
• tellite terminal refers to the terrestrial terminals, place on the Earth, which communicate with a certain satellite.
• Figure 1 shows a plurality of satellite terminals STi ST 2 ..., served by satellite 1.
• users we refer to "users” as a synonym of "satellite terminals”.
• MIMO in expressions such as “MIMO satellite scenarios”, “MIMO precoding” or “MIMO processing” refers to situations in which a plurality of input signals and a plurality of output signals is considered in order to carry out a certain activity, such as precoding or processing. These multiple inputs and multiple outputs relate to multiple satellite beams focussed to multiple satellite terminals or to multiple cells .
• transmission and its derivates (to transmit%) , referred to a precoding vector or to a precoding matrix, should be interpreted as the joint transmission of such a vector or matrix and a data vector .
• the operator (D) denotes hermitian transposition of a matrix.
• Figure 1 is a schematic representation of a forward link between a gateway, a bent pipe satellite and a plurality of satellite terminals on the Earth.
• Figure 1 shows a satellite 1, a plurality of satellite terminals ST 11 , ST 12 ... ST 21 , ST 22 , ST 23 ... ST K1 , ST K2 ... located on the Earth, and a gateway 2.
• the total number of satellite terminals is N.
• the gateway 2, which is a control station placed on the Earth, is in charge of the processing, coding, scheduling, etc. of the satellite signals.
• the satellite 1, located in the space receives a signal sent by the gateway 2 and sends it towards the Earth (towards the satellite terminals) . In other words, the satellite 1 acts as repeater.
• the gateway 2 is connected to a terrestrial network 3.
• This satellite-gateway configuration corresponds to current technological limitations, in the sense that current satellites act mostly as repeaters, while the processing capacity lies at a gateway.
• this configuration may evolve to situations wherein the satellite comprises part or all the system intelligence. This variation is obviously comprised within the scope of the invention.
• a single satellite is controlled by several gateways, each of them controlling a group of those information beams.
• a satellite can commercially generate and carry up to 106 satellite beams at the same time, while each gateway is usually designed to control the information of about 7 satellite beams.
• a gateway can be designed to manage the satellite information transmitted over a certain area.
• K satellite beams are generated to cover a certain area 4. These K beams are controlled by a single terrestrial gateway 2.
• area 4 is divided into M cells Ci, C 2 , ..., C M , each of them comprising one or more satellite terminals ST 11 , ST 12 ... ST 21 , ST 22 , ST 23 ... ST M1 , ST M2 ...
• the number of cells M is equal or greater than the number of satellite beams K (M ⁇ K) .
• M K, which means that the system provides one satellite beam for each cell C 1 .
• M > K in which situation the system temporally disable a number M - K cells, in order to have the same number of satellite beams as enabled cells.
• area 4 is formed by a single cell comprising the whole amount of N satellite terminals ST 11 , ST 12 ... ST 21 , ST 22 , ST 23 ... ST K1 , ST K2 ....
• the whole cell is served by K satellite beams.
• a MIMO satellite scenario needs to be defined. This is represented in figure 1.
• a gain precoding scheme needs to be established. This gain precoding scheme is dependent on the multiuser diversity caused by the N satellite terminals. It has to be noted that the requirement of low complexity transmission techniques for their application in commercial systems restricts the choice of said precoding scheme to low complexity ones.
• the joint processing of the method of the invention is also dependent on the channel characteristics. It is remarked that, while the processing disclosed by L. Cottatellucci, et . al . (see the referral in the "State of the Art") takes the channel into consideration, it does not exploit its specific characteristics. Its scheme is the same as that of terrestrial communications, without specific considerations to satellite systems.
• the characteristics of the satellite channel are dependent on the line of sight (LOS) .
• the coverage area of a satellite beam is related to the radiation pattern of the satellite antennas used to generate such a satellite beam.
• a single receiving antenna per satellite terminal is considered, as having more than one antenna per satellite terminal is not beneficial to the system (i.e. they are undistinguishable and thus seen as a single receiving antenna) .
• Figure 2 exhibits an example of the radiation pattern corresponding to a single beam at the satellite 1.
• the horizontal axis (X-axis) represents the angle of radiation ⁇ (in degrees °) and the vertical axis (Y-axis) represents the attenuation (dB) .
• the radiated power is normalised to 1 Watt, and therefore at the centre of the beam (zero angle (0°)) the power is 0 dB .
• the satellite antennas radiation patterns are known and therefore, the radiated power level from each satellite beam towards the physical locations on Earth can be computed.
• FIG 3 The value of the radiated power level from a satellite beam at a certain location on the Earth is illustrated in figure 3. It is obtained by first computing the radiation angle ⁇ at which a ground location (for example, satellite terminal ST 1 I) is illuminated by a beam.
• Figure 3 relates to the trigonometric relationship between a ground location
• the radiated power level information is obtained at the gateway 2 without any training transmission process.
• the satellite beams are designed according to a predefined radiation function.
• radiation functions are the Bessel functions.
• One of these Bessel functions is shown in figure 2.
• the gateway 2 knows in advance the radiation pattern. For example, as illustrated in figure 2, if a power of 1 Watt is injected to a beam, then a power of 0 dB is emitted at the zero angle (0°) . Similarly, said injected power of 0 dB implies a power of about -45 dB at the angle equal to 10°.
• a satellite terminal has the same channel characteristics with respect to all different satellite beams; only the received power level varies from beam to beam. In other words, the large distance (in the order of several thousands of Km) between a satellite and a satellite terminal makes the channel characteristics seen from a certain location or satellite terminal ST ct to all the satellite beams be the same.
• the received power at a ground location from each of the satellite beams can be determined. Therefore, generating precoding vectors in such a way that they exploit the information regarding to the antennas radiation pattern is of great help to increase the system performance. This is not guaranteed if conventional terrestrial systems are used, where precoding vectors are not only randomly generated, but also they do not take into account the line of sight channel characteristics nor the radiation pattern of the antennas.
• the present invention takes advantage of the above- mentioned channel characterisation (Line of Sight) , in order to carry out a precoding or preprocessing which takes into account the signal contribution from each of the beams. The bitrate is thus increased. Furthermore, the present invention combines this information (channel characterisation) with the multiuser diversity for designing a Multibeam Opportunistic Beamforming technique that extracts the scenario multiuser gain.
• the precoder generation of the present invention provides a low complexity design and implementation. As a random precoding generation is not a proper approach in order to extract all the benefits from the MIMO processing, by exploiting the characteristics of the satellite communication scenario, the precoding generation can benefit from the Line of Sight (LOS) channel condition. Therefore, the selection of satellite terminals is designed to be opportunistic.
• a plurality of N satellite terminals is served by gateway 2 via satellite 1.
• a set of K beams is available.
• the selection of the satellite terminal to be served in each cell is determined by a scheduler at the gateway 2.
• area 4 is divided into K cells, each of them having one or more satellite terminals.
• Each satellite terminal in area 4 receives a signal component from each one of the K beams.
• the received signal at each satellite terminal is thus the sum of the K received signals and can be expressed as follows:
• y is a scalar quantity representing the sum of the K received signals at a satellite terminal;
• z is the received noise;
• Ji 1 is the IxK channel of the i th satellite terminal, where each entry corresponds to the channel seen from the i th satellite terminal with respect to each one of the beams, showing same channel characteristics but with different received power levels. Note that obtaining the channel information Ji 1 is out of the scope of the present invention.
• F is the KxK precoding matrix that is in charge of the MIMO processing simultaneously carried over all the beams.
• the present invention provides a method for obtaining this precoding matrix. Therefore, F determines how the different components of the data vector s are combined over the beams in order to optimize transmission.
• Each column of the F matrix represents the MIMO processing carried over each satellite beam.
• CSI Channel State Information
• Having full or partial CSI depends on the system setup and on the available load in the feedback channel. If little feedback is allowed, then only the SNIR is fed back (partial channel information) , while if more feedback is allowed, then a full channel feedback of the selected satellite terminals is enabled.
• a first vector bi is desired to point towards a cell Ci included in area 4. Such point is chosen in accordance with the radiation pattern known by the scheduler of the gateway 2 (explained with reference to figures 2 and 3) .
• the Spatial Power Density (SPD) or received power level received in all the satellite terminals of cell Ci from each one of the K beams is known to the gateway 2, through the calculation proposed with relation to figure 3. Therefore, the gateway 2 knows in advance the average power (in modulo) that a satellite terminal located in cell Ci receives from each one of the K beams. However, the phase received from each beam is unknown.
• a first precoding vector is generated to be transmitted towards cell Ci. bi is a vector having size KxI.
• all satellite terminals in area 4 detect precoding vector fi and measure the received power (modulo) from it.
• each one of the plurality of satellite terminals STn, STi 2 ... in cell Ci feeds back to the scheduler of the gateway 2 its received SNR (signal-to- noise ratio) SNRn, SNRi 2 ....
• This first precoding vector fi forms the first column of precoding matrix F.
• the scheduler at the gateway 2 selects the satellite terminal (STn or ST i2 , if the cell has only two satellite terminals, as exampled by figure 1) within cell Ci with best fed back SNR value. Since the system allows for full CSI information from the selected users through the feedback link, the gateway 2 asks the selected satellite terminal to feed back its whole channel (amplitude and phase) h se ⁇ i). Note that each satellite terminal gets to know its full channel information during a standard training stage, which is out of the scope of this invention.
• h se ⁇ u is a vector IxK, each of which elements comprise modulo and phase.
• the goal is to reduce as much as possible the interference between beams.
• a blocking matrix is designed for such a target.
• f 2 is desired to precode data to be transmitted towards a second cell C 2 included in area 4.
• f 2 is developed in a similar way as that of precoding vector fi :
• the SPD received in cell C 2 from each one of the K beams is also known, as previously explained.
• b 2 is a vector having size KxI.
• the modulo of each of the K elements of this vector b 2 is directly taken from the SPD, as already explained, while the phase of each element is randomly generated.
• Blocking matrix D 2 has a first column corresponding to vector b 2 and a second column corresponding to a vector h H se ⁇ u: where h se ⁇ u represents the whole channel (amplitude and phase) of the selected satellite terminal of cell Ci.
• a power normalized transmitting precoding vector f 2 is generated by using the blocking matrix as
• f 2 is a normalised vector of KxI.
• the precoding vector f 2 is orthogonal to the channel of the satellite terminal in cell Ci.
• the interference to the selected satellite terminal in cell Ci is zero. Since the design is sequential, the selected user in Ci receives zero interference from f 2 , but not the other way round. This is called triangular cancellation of interference.
• the selected satellite terminal in the area of cell Ci avoids interference from the sequentially generated beams after its own beam (i.e. beams towards C 2 C3 C4 ... CK) •
• f 2 is sent towards cell C 2 .
• All satellite terminals in area 4 receive precoding vector f 2 and measure its received power from it.
• the satellite terminals in cell C 2 (ST 2i , ST 22 ...) feed back to the gateway 2 the received SNIR (SNIR 2 I, SNIR 22 ...) .
• the selected terminal in cell Ci receives zero power from f 2 .
• the scheduler at gateway 2 selects the satellite terminal (among ST 2 I, ST 22 -) with best fed back SNIR value in the area of cell C 2 . Since the information about the channel of the selected user (satellite terminal) is full, the scheduler asks the selected satellite terminal
• h se i( 2 ) is a vector IxK, each of which elements comprise modulo and phase.
• This process is sequentially repeated over all the beams pointing towards the remaining cells C3 C 4 ... C ⁇ ) .
• a blocking matrix D 3 is built as follows:
• b 3 is chosen in a similar way as bi and b 2 .
• a power normalized transmitting precoding vector f 3 is generated by using the blocking matrix as
• f 3 is a normalised vector of KxI. Like previously, f3 is sent towards cell C3.
• the satellite terminals in cell C 3 (ST 31 , ST 32 %) feed back to the gateway 2 the received SNIR (SNIR 31 , SNIR 32 %) . Notice that in the case of full CSI from the selected satellite terminals, the selected terminal in cell C 1 and that corresponding one in cell C 2 receive zero power from f 3 .
• the scheduler at gateway 2 selects the satellite terminal (among ST 31 , ST 32 ...) with best fed back SNIR value in the area of cell C 3 .
• the scheduler asks the selected satellite terminal (with best SNIR) to feed back its whole channel (amplitude and phase) h sel ( 3 ).
• h sel ( 3 ) is again a vector IxK.
• a matrix D p takes the form of:
• the selected satellite terminal in the area of cell Cp avoids interference from the sequentially generated precoding vectors after its own precoding vector.
• the precoding vector f p is orthogonal to the channel of the satellite terminal in precoding vectors fi... f p -i.
• fp is a normalised vector of KxI.
• the interference cancellation to the previously setup precoding vectors makes the user in Ci avoid receiving interference, while the user in C2 receives interference only from the precoding vector sent towards Ci, and it avoids the interference from the other precoding vectors (towards C3 C 4 ).
• the order of precoding is very important in this triangular interference cancellation process.
• the multiuser diversity gain is kept in the system because the phase received by each satellite terminal is different and the phase generation in the precoding vector itself is random.
• precoding vectors fi, f 2 ...fK are sequentially done. However, they are not transmitted toward the respective cells until the whole matrix F is built. This means that the best satellite terminals are not sequentially selected. On the contrary, the K selected satellite terminals (one per cell) are simultaneously selected once matrix F is built and sent by means of any multiple access scheme. This has the advantage of reducing the time dedicated to the initialisation of the transmission. This is explained next .
• a vector bi having size KxI is selected from the SPD. bi is normalized in power and thus converted into fi .
• a second transmitting precoding vector f 2 is now required.
• the objective is to have a precoding vector f 2 which interferes as little as possible with precoding vector fi .
• f 2 is developed in a similar way as that of precoding vector fi :
• the SPD received in cell C2 from each one of the K beams is also known, as previously explained.
• b2 is a vector having size KxI.
• the modulo of each of the K elements of this vector b2 is directly taken from the SPD, as already explained, while the phase of each element is randomly generated.
• a blocking matrix D 2 having dimension Kx2 is designed. Blocking matrix D 2 has a first column corresponding to vector b 2 and a second column corresponding to a vector f H i :
• blocking matrix D 2 is based on the information of the radiation pattern of the antennas at the satellite 1.
• f 2 is a normalised vector of KxI.
• f 2 is formulated to obtain orthogonal precoding vectors at the transmitter side. Through this approach, the received interference at each serviced satellite terminal is considerably decreased.
• This blocking matrix D 2 guarantees as little interference between fi and f 2 as possible, but it does not achieve absolute interference cancellation .
• a blocking matrix D3 is built as follows:
• a power normalized precoding vector f 3 is generated by using the blocking matrix as
• f 3 is a normalised vector of KxI.
• a power normalized transmitting precoding vector f p is generated by using the blocking matrix as
• f p D p [D P H D p ] "1 1 where again 1 is a vector of all zeros except the position that corresponds to the number "p" of a corresponding cell C p (in this case, it is position p) .
• f p is a normalized vector of KxI.
• precoding matrix F is obtained, a simultaneous transmission over all the beams with the whole precoding matrix F is done.
• each one of the plurality of satellite terminals at each cell feeds back to the scheduler of the gateway 2 its received SNIR (signal-to-noise-interference ratio) (SNIR 11 , SNIR 12 ... SNIR 21 , SNIR 22 ... SNIR K1 , SNIRR 2 ).
• SNIR 11 , SNIR 12 ... SNIR 21 , SNIR 22 ... SNIR K1 , SNIRR 2 SNIR (signal-to-noise-interference ratio)
• the scheduler at the gateway 2 selects, for each cell, the satellite terminal with best fed back SNIR value.
• a data transmission can start, wherein K users (the satellite terminals with best SNIR) are simultaneously served.
• the invention provides a method of assigning a precoding matrix in a broadband satellite system which takes into account the radiation pattern of the satellite antennas, the line of sight channel characteristics and the multiuser diversity caused by all the satellite terminals served by a satellite. Thus, interference is mitigated.
• the method of the present invention provides a number of advantages with respect to conventional methods. Some of these advantages are: -higher bitrate; -better availability (less outage) , which means that it satisfies the minimum detection requirement rate imposed by the terminals;
• the channel amplitude (obtained through the SPD) is more important in the precoder generation process. Since all terminals share similar amplitude characteristics within the same beam coverage while the phase is different among them, the method provides very similar rate over all the terminals in each coverage area. Hence, the rate assigned to the terminals over the consecutive time instants shows low variance over the time. This is actually an important feature in the transmitting frames design for its implementation in commercial standards.
• the present invention presents a precoder design that considers the satellite antennas beams radiation pattern in the precoder design process, so that the precoder is generated taking into account the LOS channel characteristics.
• the precoding is accomplished over all the beams managed by a single gateway where an opportunistic user selection process is performed. If the method of the invention is compared to that of the prior art: It is remarked that, while the processing disclosed by L. Cottatellucci, et . al . (see the referral in the "State of the Art") takes the channel into consideration, it does not exploit the information of the radiation pattern of the satellite antennas. This prior art disclosure is not aware of the fact that the channel has Line of Sight (LOS) , and therefore it does not exploit it.
• LOS Line of Sight
• Cottatellucci' s precoding scheme requires for full channel information.
• full channel information for all the users is impracticable.
• a random user selection is carried out. Therefore, all the multiuser gain is wasted.

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